1. Field of the Invention
The present invention relates to a cerium activated rare earth oxyhalide phosphor, a radiation image recording and reproducing method utilizing the same, and a radiation image storage panel employing the safe.
2. Description of Prior Art
It has been heretofore known that a cerium activated rare earth oxyhalide phosphor having the following formula: EQU LnOX:xCe
in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu; X is at least one halogen selected from the group consisting of Cl, Br and I; and x is a number satisfying the condition of 0&lt;x0.2, can be employed as a phosphor for a radiographic intensifying screen, since the phosphor gives an emission (spontaneous emission) in the blue light region with the maximum at the wavelength of approx. 380-400 nm when excited with a radiation such as X-rays. Recently, it has been discovered that said cerium activated rare earth oxyhalide phosphor emits light in the blue region when excited with an electromagnetic wave having a wavelength within the region of 450-900 nm after exposure to a radiation such as X-rays, that is, the phosphor gives stimulated emission. Because of the stimulability thereof, the cerium activated rare earth oxyhalide phosphor has been paid much attention and investigated as a phosphor for a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor (see: Japanese Patent Publication No. 59(1984)-44339).
The radiation image recording and reproducing method utilizing a stimulable phosphor can be employed in place of the conventional radiography utilizing a combination of a radiographic film and an intensifying screen. The method involves steps of causing a stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; sequentially exciting (or scanning) the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
The radiation image storage panel employed for the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a transparent film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to protect the phosphor layer from chemical deterioration or physical shock.
There is not only known a stimulable phosphor layer comprising a binder and a stimulable phosphor dispersed therein, but also a phosphor layer comprising agglomerated phosphor (which contains no binder), which can be formed by a sintering method or deposition method (see: U.S. Pat. No. No. 4,789,202; European Patent Application No. 87110090.5). The inventors have also applied for patent with respect to a stimulable phosphor layer of an agglomerated phosphor which is impregnated with a polymer material (U.S. Pat. application No. 184,010; European Patent Application No. 88106327.5). In any kind of the stimulable phosphor layer, the stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
When a radiation image storage panel containing a stimulable phosphor is employed in radiography for the medical diagnosis, it is particularly desired that the sensitivity of the panel to a radiation is made as high as possible to reduce the exposure dose for patient and to facilitate the procedure for converting the stimulated emission to electric signals. Accordingly, it is desired to make the luminance of stimulated emission of the phosphor employed for the panel as high as possible.
In order to make the luminance of stimulated emission of the above-described cerium activated rare earth oxyhalide phosphor high, a process for the preparation of the phosphor was proposed, which has a step of adding tetrafluoroboric acid compounds, hexafluorosilicic acid compounds or metal fluorides to a mixture of starting materials for the phosphor or a heat-treated product of said mixture before firing the mixture (U.S. Pat. No. 4,539,137).
A phosphor of GdOCl:xCe (i.e. Ln=Gd, X=Cl in the above-described formula: LnOX:xCe) in which x is a number satisfying the condition of 0&lt;x0.2 absorbs much amount of X-ray radiation, and exhibits higher luminance of stimulated emission than other cerium activated rare earth oxyhalide phosphors (see Japanese Patent Provisional Publication No. 1(1989)-95183.
The present applicant(s) applied for patent with respect to a process for the preparation of the cerium activated rare earth oxyhalide phosphor, which has a step of firing a mixture of starting materials for the phosphor or a heat-treated product of said mixture under a high pressure, in order to obtain a high-luminance cerium activated rare earth oxyhalide phosphor (U.S. patent application No. 296,212).
The cerium activated rare earth oxyhalide phosphor expressed by the above-described formula consists essentially of cerium as an activator and LnOX as a matrix crystal which has the PbFCl-type crystal structure and is composed of rare earth element Ln, oxygen O and halogen X.
The expression of LnOX in the above-described formula means that rare earth element Ln, oxygen O and halogen X together consist in a matrix crystal whose structure is same as that of PbFCl crystal, and the expression does not mean that the atomic ratio of Ln, O and X is always 1:1:1 in the crystal. However, in any of the above-mentioned references, there is disclosed neither the ratio between Ln and X in the phosphors nor the relation between the characteristics of stimulated emission and the ratio between Ln and X.
In U.S. Pat. No. 4,539,137, there is disclosed the stimulation spectrum of LaOBr:Ce phosphor, which has the maximum near 480 nm. In Japanese Patent Publication No. 59(1984)-44339, a stimulation spectrum of LaOBr:Ce,Tb having the maximum near 580 nm is shown.
The phosphors whose stimulation spectra are disclosed in the above-mentioned references exhibit high luminance of stimulated emission and are suitable for the radiation image recording and reproducing method. However, their maximum peaks of the stimulation spectra are located in shorter wavelengths (530 nm or 480 nm) than a radiation wavelength of He-Ne laser (633 nm), which is commonly used as a light source of stimulating rays, or those of semiconductor laser (680 nm, 750 nm, 780 nm, 830 nm), which is inexpensive light source and is easy to handle. Therefore, these phosphors cannot efficiently absorb the stimulating rays radiated from these lasers.
Although one can choose a light source to match with the radiation wavelength of the source having the wavelength of the maximum peak of the stimulation spectrum, lasers which radiate at shorter wavelengths than He-Ne laser or semi-conductor laser are generally expensive or are too voluminous to handle easily. Further, light sources except lasers are not commonly available for the radiation image recording and reproducing method, because the energy densities per unit area of their rays are too low to stimulate phosphors and the light scanning method using their rays is restricted.
Therefore, the cerium activated rare earth oxyhalide phosphor is desired to have the maximum peak of its stimulation spectrum at longer wavelength than those of the above-mentioned phosphors are, so as to match with the radiation wavelength of He-Ne laser or semiconductor laser.
Naturally, a phosphor which cannot store sufficient radiation energy or of which quantum yield is low does not exhibit high luminance of the stimulated emission, even if the phosphor absorbs stimulating rays efficiently. Therefore, the cerium activated rare earth oxyhalide phosphor is desired to absorb energy of stimulating rays efficiently and to exhibit high luminance of the stimulated emission.